Method for producing low molecular weight polytetrafluoroethylene, low molecular weight polytetrafluoroethylene, and powder

ABSTRACT

Powder including low molecular weight polytetrafluoroethylene having a melt viscosity of 1×102 to 7×105 Pa·s at 380° C., having a melt viscosity of 1×102 to 7×105 Pa·s at 380° C., having an average particle size of 1.0 to 50 μm, and containing 30 or more carboxyl groups at ends of the molecule chain per 106 carbon atoms in the main chain, wherein the powder is substantially free from C8-C14 perfluorocarboxylic acids and salts thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Rule 53(b) Continuation application of U.S.application Ser. No. 17/078,365 filed Oct. 23, 2020, which is a Rule53(b) Continuation of U.S. application Ser. No. 16/069,753 filed Jul.12, 2018, which is a National Stage of International Application No.PCT/JP2017/028469, filed Aug. 4, 2017 and which claims priority fromJapanese Patent Application No. 2016-153856 filed Aug. 4, 2016, therespective disclosures of all of the above of which are incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The invention relates to methods for producing low molecular weightpolytetrafluoroethylene, low molecular weight polytetrafluoroethylene,and powder.

BACKGROUND ART

Low molecular weight polytetrafluoroethylene (also referred to as“polytetrafluoroethylene wax” or “polytetrafluoroethylene micro powder”)having a molecular weight of several thousands to several hundreds ofthousands has excellent chemical stability and a very low surfaceenergy, as well as low fibrillatability. Thus, low molecular weightpolytetrafluoroethylene is used as an additive for improving thesmoothness and the texture of film surfaces in production of articlessuch as plastics, inks, cosmetics, coatings, and greases (for example,see Patent Literature 1).

Examples of known methods for producing low molecular weightpolytetrafluoroethylene include polymerization, radiolysis, andpyrolysis.

With regard to the radiolysis among these techniques, Patent Literature2 discloses a method for producing polytetrafluoroethylene powder havingan average particle size of 200 micrometers or smaller, includingirradiating polytetrafluoroethylene powder or a preformed article orarticle thereof with ionizing radiation of at least 5×10⁵ röntgen, andthen pulverizing the irradiated article.

Patent Literature 3 discloses a method for disintegratingpolytetrafluoroethylene resin, including irradiatingpolytetrafluoroethylene resin with ionizing radiation in the presence ofan oxygen component, heating the irradiated resin, and mechanicallypulverizing the heated resin.

Patent Literature 4 discloses a method for finely powderingpolytetrafluoroethylene resin, including irradiatingpolytetrafluoroethylene resin with ionizing radiation in the presence ofan oxygen component, heating the irradiated resin in the presence ofhalogenated methane and the oxygen component together, and thenmechanically pulverizing the heated resin.

CITATION LIST Patent Literature

-   Patent Literature 1: JP H10-147617 A-   Patent Literature 2: JP S47-19609 B-   Patent Literature 3: JP S51-3503 B-   Patent Literature 4: JP S52-25858 B

SUMMARY OF INVENTION Technical Problem

The inventors found that irradiation under conventional conditionsunfortunately generates short-chain perfluorocarboxylic acids or saltsthereof. The short-chain perfluorocarboxylic acids and salts thereofinclude non-naturally occurring, difficult-to-decompose substances whichare further indicated to have high bioaccumulation, i.e.,perfluorooctanoic acid containing 8 carbon atoms and salts thereof,perfluorononanoic acid containing 9 carbon atoms and salts thereof, andperfluorodecanoic acid, perfluoroundecanoic acid, perfluorododecanoicacid, perfluorotridecanoic acid, and perfluorotetradecanoic acidrespectively containing 10, 11, 12, 13, and 14 carbon atoms and saltsthereof.

In view of the above current state of the art, the invention aims toprovide a method for producing low molecular weightpolytetrafluoroethylene enabling easy removal of most of C8-C14perfluorocarboxylic acids and salts thereof, which are unfortunatelygenerated by irradiation, from the low molecular weightpolytetrafluoroethylene.

Solution to Problem

The invention relates to a method for producing low molecular weightpolytetrafluoroethylene, including: (1) irradiatingpolytetrafluoroethylene to provide low molecular weightpolytetrafluoroethylene having a melt viscosity of 1×10² to 7×10⁵ Pa·sat 380° C.; (2) pulverizing the low molecular weightpolytetrafluoroethylene; and (3) heating the low molecular weightpolytetrafluoroethylene pulverized in the step (2).

The heating is preferably performed at a temperature of 50° C. to 300°C.

The heating is also preferably performed at a temperature of 50° C. to200° C.

The polytetrafluoroethylene preferably has a standard specific gravityof 2.130 to 2.230.

Both the polytetrafluoroethylene and the low molecular weightpolytetrafluoroethylene are preferably in the form of powder.

The production method preferably further includes (4) heating thepolytetrafluoroethylene up to a temperature that is not lower than theprimary melting point thereof to provide a molded article before thestep (1), the molded article having a specific gravity of 1.0 g/cm³ orhigher.

The invention also relates to low molecular weightpolytetrafluoroethylene obtainable by the above production method.

The invention also relates to powder containing low molecular weightpolytetrafluoroethylene, the low molecular weightpolytetrafluoroethylene having a melt viscosity of 1×10² to 7×10⁵ Pa·sat 380° C., having an average particle size of 1.0 to 50 μm, andcontaining 30 or more carboxyl groups at ends of the molecule chain per10⁶ carbon atoms in the main chain, the powder being substantially freefrom C8-C14 perfluorocarboxylic acids and salts thereof.

The powder preferably satisfies that the perfluorocarboxylic acids andsalts thereof amount to less than 25 ppb.

The powder preferably has a specific surface area of 0.5 to 20 m²/g.

The powder also preferably has a specific surface area of 7.0 to 20m²/g.

Advantageous Effects of Invention

The production method of the invention enables easy removal of most ofC8-C14 perfluorocarboxylic acids and salts thereof, which areunfortunately generated by irradiation, from low molecular weightpolytetrafluoroethylene.

DESCRIPTION OF EMBODIMENTS

The invention will be specifically described hereinbelow.

The production method of the invention includes: (1) irradiatingpolytetrafluoroethylene (PTFE) to provide low molecular weight PTFEhaving a melt viscosity of 1×10² to 7×10⁵ Pa·s at 380° C.; (2)pulverizing the low molecular weight PTFE; and (3) heating the lowmolecular weight PTFE pulverized in the step (2).

In the step (1), the PTFE can be irradiated by a conventionally knownmethod under conventionally known conditions. Irradiation of the PTFEunder conventional irradiating conditions generates low molecular weightPTFE having a higher melt viscosity than the PTFE, as well as C8-C14perfluorocarboxylic acids and salts thereof. Such perfluorocarboxylicacids and salts thereof can be removed from the low molecular weightPTFE by pulverizing the low molecular weight PTFE and then heating thepulverized low molecular weight PTFE after the irradiation.

Examples of the radiation include any ionizing radiation, such aselectron beams, ultraviolet rays, gamma rays, X-rays, neutron beams, andhigh energy ions. Electron beams or gamma rays are preferred.

The radiation preferably has an exposure dose of 1 to 2500 kGy, morepreferably 1000 kGy or lower, still more preferably 750 kGy or lower,while more preferably 10 kGy or higher, still more preferably 100 kGy orhigher.

The irradiation temperature may be any temperature within the range of5° C. to the melting point of PTFE. It is known that the molecule chainof PTFE is crosslinked around the melting point thereof. The irradiationtemperature is therefore preferably 320° C. or lower, more preferably300° C. or lower, still more preferably 260° C. or lower, in order toprovide low molecular weight PTFE. From an economic viewpoint, theirradiation is preferably performed at room temperature.

In the step (1), the irradiation may be performed in any atmosphere,such as in the air, inert gas, or vacuum. In order to reduce the cost,the irradiation is preferably performed in the air. In order to reducegeneration of C8-C14 perfluorocarboxylic acids and salts thereof, theirradiation is preferably performed substantially in the absence ofoxygen. It should be noted that irradiation substantially in the absenceof oxygen is not essential because the production method of theinvention includes the pulverization and the heating after theirradiation.

The step (1) preferably provides particles of the low molecular weightPTFE having an average particle size of 500 μm or smaller. The averageparticle size of the low molecular weight PTFE particles is morepreferably 300 μm or smaller, still more preferably 100 μm or smaller.The lower limit thereof may be, but is not limited to, greater than 30μm. Low molecular weight PTFE particles having an average particle sizewithin the above range can easily provide powder of low molecular weightPTFE having a relatively small average particle size.

The average particle size is equivalent to the particle sizecorresponding to 50% of the cumulative volume in the particle sizedistribution determined using a laser diffraction particle sizedistribution analyzer (HELOS & RODOS) available from Jeol Ltd. at adispersive pressure of 1.0 bar without cascade impaction.

The pulverization in the step (2) may be performed by any method, suchas pulverization using a pulverizer. Examples of the pulverizer includeimpact-type pulverizers such as hammer mills, pin mills, and jet mills,and grinding-type pulverizers utilizing shearing force generated byunevenness between a rotary blade and a peripheral stator, such ascutter mills.

The pulverization temperature is preferably not lower than −200° C. butlower than 50° C. In the case of freeze pulverization, the pulverizationtemperature is usually −200° C. to −100° C. Still, the pulverization maybe performed around room temperature (10° C. to 30° C.). Freezepulverization is usually achieved by the use of liquid nitrogen, butsuch pulverization requires enormous equipment and high pulverizationcost. In order to simplify the step and reduce the pulverization cost,the pulverization temperature is more preferably not lower than 10° C.but lower than 50° C., still more preferably 10° C. to 40° C.,particularly preferably 10° C. to 30° C.

The pulverization may be followed by removal of particles and fibrousparticles by air classification, and further followed by removal ofcoarse particles by classification.

In the air classification, the pulverized particles are sent to acylindrical classification chamber by decompressed air, dispersed byswirl flow inside the chamber, and classified by centrifugal force. Theparticles are collected from the central portion into a cyclone and abag filter. Inside the classification chamber is provided a rotarydevice such as a circular-cone-like cone or rotor configured to achievehomogeneous gyrating movement of the pulverized particles and the air.

In the case of using a classification cone, the classification point isadjusted by controlling the volume of the secondary air or the gap fromthe classification cone. In the case of using a rotor, the air volumeinside the classification chamber is adjusted by the number of rotationsof the rotor.

Examples of the method of removing coarse particles include airclassification, vibration sieving with meshes, and ultrasonic sievingwith meshes. Air classification is preferred.

The step (2) provides pulverized particles of the low molecular weightPTFE having a smaller average particle size than the low molecularweight PTFE particles obtained in the step (1). Preferably, thepulverized particles of the low molecular weight PTFE have an averageparticle size of 1 to 200 μm. The average particle size of thepulverized particles of the low molecular weight PTFE is more preferably100 μm or smaller, and may be 1.0 to 50 μm. Pulverized particles of thelow molecular weight PTFE having an average particle size within theabove range can easily provide powder of low molecular weight PTFEhaving a relatively small average particle size.

The average particle size is equivalent to the particle sizecorresponding to 50% of the cumulative volume in the particle sizedistribution determined using a laser diffraction particle sizedistribution analyzer (HELOS & RODOS) available from Jeol Ltd. at adispersive pressure of 1.0 bar without cascade impaction.

The heating in the step (3) is preferably performed at a temperaturehigher than the pulverization temperature in the step (2), and ispreferably performed at 50° C. to 300° C., for example. The heatingtemperature is more preferably 70° C. or higher, still more preferably90° C. or higher, particularly preferably 100° C. or higher, while morepreferably 230° C. or lower, still more preferably 200° C. or lower,particularly preferably 130° C. or lower. Too low a heating temperaturemay cause insufficient removal of C8-C14 perfluorocarboxylic acids andsalts thereof. Too high a heating temperature may cause disadvantagessuch as a failure in achieving effects that correspond to the energyrequired for the heating, agglomeration of powder, and deformation ofparticles.

The heating may be performed for any duration, and the heating durationis preferably 10 seconds to 5 hours, more preferably 5 minutes orlonger, still more preferably 10 minutes or longer, while morepreferably 4 hours or shorter, still more preferably 3 hours or shorter.Too short a heating duration may cause insufficient removal of C8-C14perfluorocarboxylic acids and salts thereof. Too long a heating durationmay cause disadvantages such as a failure in achieving effects thatcorrespond to the heating duration, agglomeration of powder, anddeformation of particles.

The heating may be performed by any method, such as methods using any ofthe following heating devices. Examples of the heating devices includebox dryers, band dryers, tunnel dryers, nozzle jet dryers, moving-beddryers, rotary dryers, fluidized-bed dryers, air-flow dryers, boxdryers, disc dryers, cylindrical mixing dryers, inverted-cone mixingdryers, microwave devices, vacuum heaters, box electric furnaces,hot-air circulating devices, flash dryers, vibration dryers, beltdryers, extrusion dryers, and spray dryers.

In the step (3), the heating may be performed in any atmosphere. Fromthe viewpoints of safety and economy, the heating is preferablyperformed in the air.

In the step (3), the heating may be performed by placing the lowmolecular weight PTFE in a heating furnace, increasing the temperatureinside the heating furnace up to a predetermined temperature, and thenleaving the PTFE for a predetermined period of time.

As described above, the low molecular weight PTFE is heated after thepulverization in the production method of the invention. Thus, most ofC8-C14 perfluorocarboxylic acids and salts thereof unfortunatelygenerated during the irradiation can be removed from the low molecularweight PTFE. If the order of the pulverization and the heating isreversed, C8-C14 perfluorocarboxylic acids and salts thereof cannot besufficiently removed.

The production method of the invention may further include (4) heatingthe PTFE up to a temperature that is not lower than the primary meltingpoint thereof to provide a molded article before the step (1). In thiscase, the molded article obtained in the step (4) can be used as thePTFE in the step (1). The primary melting point is preferably 300° C. orhigher, more preferably 310° C. or higher, still more preferably 320° C.or higher. The primary melting point means the maximum peak temperatureon an endothermic curve present on the crystal melting curve whenunsintered PTFE is analyzed with a differential scanning calorimeter.The endothermic curve is obtainable by increasing the temperature at atemperature-increasing rate of 10° C./min using a differential scanningcalorimeter.

The molded article in the step (4) preferably has a specific gravity of1.0 g/cm³ or higher, more preferably 1.5 g/cm³ or higher, whilepreferably 2.5 g/cm³ or lower.

The specific gravity can be determined by water displacement.

The production method of the invention may further include pulverizingthe molded article to provide powder of the PTFE after the step (4). Themolded article may be first coarsely and then finely pulverized.

The production method of the invention may further include (5)pulverizing the low molecular weight PTFE heated in the step (3). Thiscan easily provide powder of low molecular weight PTFE having a muchsmaller average particle size.

Next, the PTFE to be irradiated and the low molecular weight PTFE to beobtained after the irradiation in the production method of the inventionare described hereinbelow.

The low molecular weight PTFE has a melt viscosity of 1×10² to 7×10⁵Pa·s at 380° C. The term “low molecular weight” herein means that themelt viscosity is within this range.

The melt viscosity is a value determined by heating a 2-g sample at 380°C. for five minutes in advance and then keeping this sample at thistemperature under a load of 0.7 MPa using a flow tester (Shimadzu Corp.)and a 2ϕ-8 L die in conformity with ASTM D1238.

The PTFE to be irradiated preferably has a standard specific gravity(SSG) of 2.130 to 2.230. The standard specific gravity (SSG) is a valuedetermined in conformity with ASTM D4895.

The PTFE has a significantly higher melt viscosity than the lowmolecular weight PTFE, and thus the melt viscosity thereof is difficultto measure accurately. In contrast, the melt viscosity of the lowmolecular weight PTFE is measurable, but the low molecular weight PTFEhas difficulty in providing a molded article usable for measurement ofstandard specific gravity. Thus, the standard specific gravity thereofis difficult to measure accurately. Therefore, in the invention, thestandard specific gravity is used as an indicator of the molecularweight of the PTFE to be irradiated, while the melt viscosity is used asan indicator of the molecular weight of the low molecular weight PTFE.For both the PTFE and the low molecular weight PTFE, no method fordetermining the molecular weight directly has been known so far.

The low molecular weight PTFE preferably has a melting point of 324° C.to 336° C.

The melting point is defined using a differential scanning calorimeter(DSC) as follows. Specifically, temperature calibration is performed inadvance with indium and lead as standard samples. Then, about 3 mg oflow molecular weight PTFE is put into an aluminum pan (crimpedcontainer), and the temperature is increased at a rate of 10° C./minwithin the temperature range of 250° C. to 380° C. under air flow at 200ml/min. The minimum point of the heat of fusion within this region isdefined as the melting point.

In the production method of the invention, the PTFE may be in any form,such as powder, a molded article of the PTFE, or shavings generated bycutting the molded article of the PTFE. The PTFE in the form of powdercan easily provide powder of the low molecular weight PTFE.

The low molecular weight PTFE obtainable by the production method of theinvention may be in any form, and is preferably in the form of powder.

When the low molecular weight PTFE obtainable by the production methodof the invention is in the form of powder, the specific surface areathereof is preferably 0.5 to 20 m²/g. The specific surface area is morepreferably 7.0 m²/g or larger.

For the low molecular weight PTFE powder, both of the following twotypes are demanded, i.e., a small specific surface area type having aspecific surface area of not smaller than 0.5 m²/g but smaller than 7.0m²/g and a large specific surface area type having a specific surfacearea of not smaller than 7.0 m²/g and not larger than 20 m²/g.

The low molecular weight PTFE powder of a small specific surface areatype has an advantage of easy dispersion in a matrix material such ascoating. In contrast, such powder disperses in a matrix material with alarge dispersed particle size, i.e., with poor fine dispersibility.

The low molecular weight PTFE powder of a small specific surface areatype preferably has a specific surface area of 1.0 m²/g or larger, whilepreferably 5.0 m²/g or smaller, more preferably 3.0 m²/g or smaller.Suitable examples of the matrix material include plastics and inks, aswell as coatings.

The low molecular weight PTFE powder of a large specific surface areatype, when dispersed in a matrix material such as coating, hasadvantages of high surface-modifying effects, such as a small dispersedparticle size in a matrix material and improved texture of the filmsurface, and a large amount of oil absorption. In contrast, such powdermay not be easily dispersed in a matrix material (e.g., take a long timefor dispersion), and may cause an increased viscosity of coating, forexample.

The low molecular weight PTFE powder of a large specific surface areatype preferably has a specific surface area of 8.0 m²/g or larger, whilepreferably 15 m²/g or smaller, more preferably 13 m²/g or smaller.Suitable examples of the matrix material include oils, greases, andcoatings, as well as plastics.

The specific surface area is determined by the BET method using asurface analyzer (trade name: BELSORP-mini II, MicrotracBEL Corp.), agas mixture of 30% nitrogen and 70% helium as carrier gas, and liquidnitrogen for cooling.

When the low molecular weight PTFE obtainable by the production methodof the invention is in the form of powder, the average particle sizethereof is preferably 0.5 to 200 μm, more preferably 50 μm or smaller,still more preferably 20 μm or smaller, particularly preferably 10 μm orsmaller, further more preferably 5 μm or smaller. The lower limitthereof may be 1.0 μm. As mentioned here, powder having a relativelysmall average particle size, when used as an additive for coating, forexample, can provide a film having much better surface smoothness.

The average particle size is equivalent to the particle sizecorresponding to 50% of the cumulative volume in the particle sizedistribution determined using a laser diffraction particle sizedistribution analyzer (HELOS & RODOS) available from Jeol Ltd. at adispersive pressure of 1.0 bar without cascade impaction.

The production method of the invention can provide low molecular weightPTFE containing hardly any C8-C14 perfluorocarboxylic acids and saltsthereof after the step (3). The low molecular weight PTFE obtainable bythe production method of the invention preferably contains C8-C14perfluorocarboxylic acids and salts thereof in a total amount by mass ofnot more than 50 ppb, more preferably less than 25 ppb, still morepreferably not more than 15 ppb, particularly preferably not more than 5ppb, most preferably less than 5 ppb. The lower limit of the amount maybe any value, and may be lower than the detection limit.

The amount of the perfluorocarboxylic acids and salts thereof can bedetermined by liquid chromatography.

The low molecular weight PTFE obtainable by the production method of theinvention is also characterized in that it contains hardly anyperfluorooctanoic acid and salts thereof. The low molecular weight PTFEobtainable by the production method of the invention preferably containsperfluorooctanoic acid and salts thereof in an amount by mass of lessthan 25 ppb. This amount is more preferably not more than 15 ppb, stillmore preferably not more than 5 ppb, particularly preferably less than 5ppb. The lower limit may be any value, and may be lower than thedetection limit.

The amount of perfluorooctanoic acid and salts thereof can be determinedby liquid chromatography.

The invention also relates to low molecular weight PTFE obtainable bythe aforementioned production method. The low molecular weight PTFE ofthe invention contains hardly any C8-C14 perfluorocarboxylic acids andsalts thereof. The low molecular weight PTFE of the invention preferablycontains C8-C14 perfluorocarboxylic acids and salts thereof in a totalamount by mass of not more than 50 ppb, more preferably less than 25ppb, still more preferably not more than 15 ppb, particularly preferablynot more than 5 ppb, most preferably less than 5 ppb. The lower limitthereof may be any value, and may be lower than the detection limit.

The low molecular weight PTFE of the invention preferably containsperfluorooctanoic acid and salts thereof in an amount by mass of lessthan 25 ppb, more preferably not more than 15 ppb, still more preferablynot more than 5 ppb, particularly preferably less than 5 ppb. The lowerlimit thereof may be any value, and may be lower than the detectionlimit.

The low molecular weight PTFE of the invention may be in any form, andis preferably in the form of powder.

When the low molecular weight PTFE of the invention is in the form ofpowder, the specific surface area thereof is preferably 0.5 to 20 m²/g.The specific surface area is more preferably 7.0 m²/g or larger.

When the low molecular weight PTFE of the invention is in the form ofpowder, the average particle size thereof is preferably 1.0 to 200 μm,more preferably 20 μm or smaller, still more preferably 10 μm orsmaller, particularly preferably 5 μm or smaller. As mentioned here,powder having a relatively small average particle size, when used as anadditive for coating, for example, can provide a film having much bettersurface smoothness.

The low molecular weight PTFE preferably contains 30 or more carboxylgroups at ends of the molecule chain per 10⁶ carbon atoms in the mainchain. The number of carboxyl groups is more preferably 35 or more per10⁶ carbon atoms in the main chain. The upper limit of the number ofcarboxyl groups may be any value, and is preferably 500, more preferably350, per 10⁶ carbon atoms in the main chain, for example. The carboxylgroups may be generated at ends of the molecule chain of the lowmolecular weight PTFE by the aforementioned irradiation of the PTFE inthe presence of oxygen, for example. The number of carboxyl groups afterirradiation increases in accordance with the amount of modification inthe PTFE. As the low molecular weight PTFE contains 30 or more carboxylgroups at ends of the molecule chain per 10⁶ carbon atoms in the mainchain, it can have excellent dispersibility in molding materials, inks,cosmetics, coatings, greases, components for office automation devices,toner-modifying additives, additives for plating solutions, and others.For example, micro powder is blended into hydrocarbon-based matrixresins, inks, and coatings for the purpose of improving the slidability,reducing the abrasion loss, preventing squeal, and improving the waterand oil repellency. However, such micro powder, which is aperfluororesin, is originally poor in compatibility with matrix resins,inks, and coatings, and thus is difficult to disperse uniformly. Incontrast, micro powder produced by irradiating and decomposing highmolecular weight PTFE generates perfluorooctanoic acid (PFOA) and saltsthereof and carboxyl groups as by-products due to the production methodthereof. Carboxyl groups present at ends and other positions in theresulting micro powder consequently act as dispersants forhydrocarbon-based matrix resin, inks, and coatings.

The low molecular weight PTFE may contain, at ends of the moleculechain, unstable end groups derived from the chemical structure of apolymerization initiator or chain-transfer agent used in thepolymerization reaction of PTFE. Examples of the unstable end groupsinclude, but are not limited to, —CH₂OH, —COOH, and —COOCH₃.

The low molecular weight PTFE may undergo stabilization of the unstableend groups. The unstable end groups may be stabilized by any method,such as a method of exposing the unstable end groups tofluorine-containing gas to convert them into trifluoromethyl groups(—CF₃), for example.

The low molecular weight PTFE may contain amidated ends. The endamidation may be performed by any method, such as a method of bringingfluorocarbonyl groups (—COF) obtained by exposure to fluorine-containinggas into contact with ammonia gas as disclosed in JP H04-20507 A, forexample.

The low molecular weight PTFE with stabilization or end amidation of theunstable end groups as described above can be well compatible withopposite materials and have improved dispersibility when used as anadditive for opposite materials such as coatings, greases, cosmetics,plating solutions, toners, and plastics.

The PTFE may be a homo-PTFE consisting only of a tetrafluoroethylene(TFE) unit or may be a modified PTFE containing a TFE unit and amodifying monomer unit based on a modifying monomer copolymerizable withTFE. In the production method of the invention, the composition of thepolymer is not changed. Thus, the low molecular weight PTFE has thecomposition of the PTFE as it is.

In the modified PTFE, the proportion of the modifying monomer unit ispreferably 0.001 to 1 mass %, more preferably 0.01 mass % or more, whilemore preferably 0.5 mass % or less, still more preferably 0.1 mass % orless, of all the monomer units. The term “modifying monomer unit” hereinmeans a moiety that is part of the molecular structure of the modifiedPTFE and is derived from a modifying monomer. The term “all the monomerunits” herein means all the moieties derived from monomers in themolecular structure of the modified PTFE. The proportion of themodifying monomer unit can be determined by any known method such asFourier transform infrared spectroscopy (FT-IR).

The modifying monomer may be any one copolymerizable with TFE, andexamples thereof include perfluoroolefins such as hexafluoropropylene(HFP); chlorofluoroolefins such as chlorotrifluoroethylene (CTFE);hydrogen-containing fluoroolefins such as trifluoroethylene andvinylidene fluoride (VDF); perfluorovinyl ether;perfluoroalkylethylenes; and ethylene. One modifying monomer may beused, or multiple modifying monomers may be used.

Examples of the perfluorovinyl ether include, but are not limited to,unsaturated perfluoro compounds represented by the following formula(1):

CF₂═CF—ORf  (1)

wherein Rf is a perfluoroorganic group. The “perfluoroorganic group”herein means an organic group in which all the hydrogen atoms bonded toany carbon atom are replaced by fluorine atoms. The perfluoroorganicgroup may contain ether oxygen.

Examples of the perfluorovinyl ether include perfluoro(alkyl vinylethers) (PAVES) represented by the formula (1) in which Rf is a C1-C10perfluoroalkyl group. The perfluoroalkyl group preferably contains 1 to5 carbon atoms.

Examples of the perfluoroalkyl group in the PAVE includeperfluoromethyl, perfluoroethyl, perfluoropropyl, perfluorobutyl,perfluoropentyl, and perfluorohexyl groups. Preferred isperfluoro(propyl vinyl ether) (PPVE) in which the perfluoroalkyl groupis a perfluoropropyl group.

Examples of the perfluorovinyl ether also include those represented bythe formula (1) in which Rf is a C4-C9 perfluoro(alkoxyalkyl) group,those represented by the formula (1) in which Rf is a group representedby the following formula:

(wherein m is 0 or an integer of 1 to 4), and those represented by theformula (1) in which Rf is a group represented by the following formula:

wherein n is an integer of 1 to 4.

Examples of the perfluoroalkylethylenes include, but are not limited to,(perfluorobutyl)ethylene (PFBE), (perfluorohexyl)ethylene, and(perfluorooctyl)ethylene.

The modifying monomer in the modified PTFE is preferably at least oneselected from the group consisting of HFP, CTFE, VDF, PPVE, PFBE, andethylene. It is more preferably at least one selected from the groupconsisting of PPVE, HFP, and CTFE.

The invention also relates to powder containing low molecular weightPTFE, the low molecular weight PTFE having a melt viscosity of 1×10² to7×10⁵ Pa·s at 380° C., having an average particle size of 1.0 to 50 μm,and containing 30 or more carboxyl groups at ends of the molecule chainper 10⁶ carbon atoms in the main chain, the powder being substantiallyfree from C8-C14 perfluorocarboxylic acids and salts thereof.

The powder of the invention is substantially free from C8-C14perfluorocarboxylic acids and salts thereof. The phrase “substantiallyfree from C8-C14 perfluorocarboxylic acids and salts thereof” hereinpreferably means that the powder contains C8-C14 perfluorocarboxylicacids and salts thereof in a total amount by mass of not more than 50ppb. The total amount thereof is more preferably less than 25 ppb, stillmore preferably not more than 15 ppb, particularly preferably not morethan 5 ppb, most preferably less than 5 ppb. The lower limit thereof maybe any value, and may be lower than the detection limit. The number ofcarbon atoms may be 10 or less.

The powder of the invention preferably contains perfluorooctanoic acidand salts thereof in a total amount by mass of less than 25 ppb, morepreferably not more than 15 ppb, still more preferably not more than 5ppb, particularly preferably less than 5 ppb. The lower limit thereofmay be any value, and may be lower than the detection limit.

The powder of the invention preferably has a specific surface area of0.5 to 20 m²/g. The specific surface area is more preferably 7.0 m²/g orlarger.

The powder of the invention has an average particle size of 1.0 to 50μm. The average particle size is preferably 20 μm or smaller, morepreferably 10 μm or smaller, still more preferably 5 μm or smaller. Asmentioned here, powder having a relatively small average particle size,when used as an additive for coating, for example, can provide a filmhaving much better surface smoothness.

For the low molecular weight PTFE constituting the powder of theinvention, the composition, melt viscosity, and specifications ofcarboxyl groups at ends of the molecule chain are similar to thosedescribed for the low molecular weight PTFE obtainable by the productionmethod of the invention.

The low molecular weight PTFE constituting the powder of the inventionmay contain unstable end groups at ends of the molecule chain, and theseunstable end groups may be stabilized, end-amidated, or end-fluorinated.These embodiments are also similar to those described for the lowmolecular weight PTFE obtainable by the production method of theinvention.

The powder of the invention may be obtainable by producing powdery lowmolecular weight PTFE by the aforementioned production method of theinvention.

The low molecular weight PTFE and the powder can suitably be used asmolding materials, inks, cosmetics, coatings, greases, components foroffice automation devices, additives for modifying toners, and additivesfor plating solutions, for example. Examples of the molding materialsinclude engineering plastics such as polyoxybenzoyl polyester,polyimide, polyamide, polyamide-imide, polyacetal, polycarbonate, andpolyphenylene sulfide. The low molecular weight PTFE is particularlysuitable as a thickening agent for greases.

The low molecular weight PTFE and the powder can be used as additivesfor molding materials for improving the non-adhesiveness and slidabilityof rollers of copiers, for improving the texture of molded articles ofengineering plastics, such as surface sheets of furniture, dashboards ofautomobiles, and covers of home appliances, and for improving thesmoothness and abrasion resistance of machine elements generatingmechanical friction, such as light-load bearings, gears, cams, buttonsof push-button telephones, movie projectors, camera components, andsliding materials.

The low molecular weight PTFE and the powder can be used as additivesfor coatings for the purpose of improving the smoothness of varnish andpaint. The low molecular weight PTFE and the powder can be used asadditives for cosmetics for the purpose of improving the smoothness ofcosmetics such as foundation.

The low molecular weight PTFE and the powder can also be suitably usedfor improving the oil or water repellency of wax and for improving thesmoothness of greases and toners.

The low molecular weight PTFE and the powder can be used as electrodebinders of secondary batteries and fuel cells, hardness adjusters forelectrode binders, and water repellents for electrode surfaces.

The low molecular weight PTFE or the powder may be combined with alubricant to provide grease. The grease is characterized by containingthe low molecular weight PTFE or the powder and a lubricant. Thus, thelow molecular weight PTFE or the powder is uniformly and stablydispersed in the lubricant and the grease exhibits excellent performancesuch as load resistance, electric insulation, and low moistureabsorption.

The lubricant (base oil) may be either mineral oil or synthetic oil.Examples of the lubricant (base oil) include paraffinic or naphthenicmineral oils, and synthetic oils such as synthetic hydrocarbon oils,ester oils, fluorine oils, and silicone oils. In terms of heatresistance, fluorine oils are preferred. Examples of the fluorine oilsinclude perfluoropolyether oil and polychlorotrifluoroethylene with alow polymerization degree. The polychlorotrifluoroethylene with a lowpolymerization degree may have a weight average molecular weight of 500to 1200.

The grease may further contain a thickening agent. Examples of thethickening agent include metal soaps, composite metal soaps, bentonite,phthalocyanin, silica gel, urea compounds, urea/urethane compounds,urethane compounds, and imide compounds. Examples of the metal soapsinclude sodium soap, calcium soap, aluminum soap, and lithium soap.Examples of the urea compounds, urea/urethane compounds, and urethanecompounds include diurea compounds, triurea compounds, tetraureacompounds, other polyurea compounds, urea/urethane compounds, diurethanecompounds, and mixtures thereof.

The grease preferably contains the low molecular weight PTFE or thepowder in an amount of 0.1 to 50 mass %, more preferably 0.5 mass % ormore and 30 mass % or less. A grease containing too large an amount ofthe low molecular weight PTFE or powder may be too hard to providesufficient lubrication. A grease containing too small an amount of thelow molecular weight PTFE or powder may fail to exert the sealability.

The grease may also contain any of additives such as solid lubricants,extreme pressure agents, antioxidants, oilness agents, anticorrosives,viscosity index improvers, and detergent dispersants.

EXAMPLES

The invention is described below with reference to examples. Still, theinvention is not intended to be limited to the following examples.

The parameters in the examples were determined by the following methods.

Average Particle Size

The average particle size was defined as the particle size correspondingto 50% of the cumulative volume in the particle size distributiondetermined using a laser diffraction particle size distribution analyzer(HELOS & RODOS) available from Jeol Ltd. at a dispersive pressure of 1.0bar without cascade impaction.

Melt Viscosity

The melt viscosity was determined by heating a 2-g sample at 380° C. forfive minutes in advance and then keeping this sample at this temperatureunder a load of 0.7 MPa using a flow tester (Shimadzu Corp.) and a 2ϕ-8L die in conformity with ASTM D1238.

Melting Point

The melting point was defined using a differential scanning calorimeter(DSC) as follows. Specifically, temperature calibration was performed inadvance with indium and lead as standard samples. Then, about 3 mg oflow molecular weight PTFE was put into an aluminum pan (crimpedcontainer), and the temperature was increased at a rate of 10° C./minwithin the temperature range of 250° C. to 380° C. under air flow at 200ml/min. The minimum point of the heat of fusion within this region wasdefined as the melting point.

Specific Surface Area

The specific surface area was determined by the BET method using asurface analyzer (trade name: BELSORP-mini II, MicrotracBEL Corp.). Agas mixture of 30% nitrogen and 70% helium was used as carrier gas, andliquid nitrogen was used for cooling.

Number of Carboxyl End Groups

The following measurement was performed in conformity with the method ofanalyzing end groups disclosed in JP H04-20507 A.

Low molecular weight PTFE powder was preformed with a hand press toprovide a film having a thickness of about 0.1 mm. The resulting filmwas subjected to infrared absorption spectrum analysis. PTFE withcompletely fluorinated ends by contact with fluorine gas was alsosubjected to infrared absorption spectrum analysis. Based on thedifference spectrum therebetween, the number of carboxyl end groups wascalculated by the following formula.

Number of carboxyl end groups (per 10⁶ carbon atoms)=(l×K)/t

l: absorbance

K: correction coefficient

t: film thickness (mm)

The absorption frequency and correction coefficient of the carboxylgroup are respectively set to 3560 cm⁻¹ and 440.

Amount of Perfluorooctanoic Acid and Salts Thereof

The amount of perfluorooctanoic acid and salts thereof was determinedusing a liquid chromatography-mass spectrometer (LC-MS ACQUITY UPLC/TQD,Waters). Measurement powder (1 g) was mixed with acetonitrile (5 ml) andthe mixture was sonicated for 60 minutes, so that perfluorooctanoic acidwas extracted. The resulting liquid phase was analyzed by multiplereaction monitoring (MRM). Acetonitrile (A) and an aqueous ammoniumacetate solution (20 mmol/L) (B) were passed at a predeterminedconcentration gradient (A/B=40/60 for 2 min and 80/20 for 1 min) asmobile phases. A separation column (ACQUITY UPLC BEH C18 1.7 μm) wasused at a column temperature of 40° C. and an injection volume of 5 μL.Electrospray ionization (ESI) in a negative mode was used as theionization method, and the cone voltage was set to 25 V. The ratio ofthe molecular weight of precursor ions to the molecular weight ofproduct ions was measured to be 413/369. The amount of perfluorooctanoicacid and salts thereof was calculated by the external standard method.The detection limit of this measurement is 5 ppb.

Amount of C8-C14 Perfluorocarboxylic Acids and Salts Thereof

C8-C14 perfluorocarboxylic acids and salts thereof were detected using aliquid chromatography-mass spectrometer (LC-MS ACQUITY UPLC/TQD,Waters). The solution used was the liquid phase extracted in themeasurement of perfluorooctanoic acid, and the measurement was performedby MRM. The measurement conditions were based on the measurementconditions for perfluorooctanoic acid, but the concentration gradientwas changed (A/B=10/90 for 1.5 min and 90/10 for 3.5 min). The ratio ofthe molecular weight of precursor ions to the molecular weight ofproduct ions was measured to be 413/369 for perfluorooctanoic acid (C8),463/419 for perfluorononanoic acid (C9), and 513/469 forperfluorodecanoic acid (C10). In the same manner, the ratio was measuredto be 563/519 for perfluoroundecanoic acid (C11), 613/569 forperfluorododecanoic acid (C12), 663/619 for perfluorotridecanoic acid(C13), and 713/669 for perfluorotetradecanoic acid (C14).

The total amount of C8-C14 perfluorocarboxylic acids was calculated fromthe amount (X) of the perfluorooctanoic acid obtained in the abovemeasurement by the following formula. The detection limit of thismeasurement is 5 ppb.

(A _(C8) +A _(C9) +A _(C10) +A _(C11) +A _(C12) +A _(C13) +A _(C14))/A_(C8) ×X

A_(C8): peak area of perfluorooctanoic acid

A_(C9): peak area of perfluorononanoic acid

A_(C10): peak area of perfluorodecanoic acid

A_(C11): peak area of perfluoroundecanoic acid

A_(C12): peak area of perfluorododecanoic acid

A_(C13): peak area of perfluorotridecanoic acid

A_(C14): peak area of perfluorotetradecanoic acid

X: amount of perfluorooctanoic acid calculated from the MRM measurementresult by the external standard method

Comparative Example 1

Commercially available homo-PTFE fine powder (standard specific gravitymeasured in conformity with ASTM D4895: 2.175) was irradiated with 150kGy of cobalt-60 γ-rays at room temperature in the air. Thereby, a lowmolecular weight PTFE powder A having an average particle size of 51.2μm was obtained.

The physical properties of the resulting low molecular weight PTFEpowder A were determined. The results are shown in Table 1.

For the low molecular weight PTFE powder A, the number of carboxyl endgroups was counted by infrared spectroscopy to be 36 groups per 10⁶carbon atoms in the main chain.

Comparative Example 2

The low molecular weight PTFE powder A obtained in Comparative Example 1was pulverized using a pulverizer. Thereby, a low molecular weight PTFEpowder B having an average particle size of 11.2 μm was obtained.

The physical properties were determined in the same manner as inComparative Example 1. The results are shown in Table 1.

Comparative Example 3

The low molecular weight PTFE powder A obtained in Comparative Example 1was heated at 100° C. for 30 minutes using a hot-air-circulatingelectric furnace (Ultra-high temperature chamber STPH-202M, EspecCorp.). Thereby, a low molecular weight PTFE powder C was obtained.

The physical properties were determined in the same manner as inComparative Example 1. The results are shown in Table 1.

Comparative Example 4

The low molecular weight PTFE powder C obtained in Comparative Example 3was pulverized using a pulverizer. Thereby, a low molecular weight PTFEpowder D was obtained.

The physical properties were determined in the same manner as inComparative Example 1. The results are shown in Table 1.

Example 1

The low molecular weight PTFE powder B obtained in Comparative Example 2was heated at 100° C. for 30 minutes using a hot-air-circulatingelectric furnace (Ultra-high temperature chamber STPH-202M, EspecCorp.). Thereby, a low molecular weight PTFE powder E was obtained.

The physical properties were determined in the same manner as inComparative Example 1. The results are shown in Table 1.

Example 2

The low molecular weight PTFE powder E obtained in Example 1 was furtherpulverized using a pulverizer. Thereby, a low molecular weight PTFEpowder F was obtained.

The physical properties were determined in the same manner as inComparative Example 1. The results are shown in Table 1.

Comparative Example 5

The low molecular weight PTFE powder A obtained in Comparative Example 1was pulverized using a pulverizer.

Thereby, a low molecular weight PTFE powder G having an average particlesize of 2.2 μm was obtained.

The physical properties were determined in the same manner as inComparative Example 1. The results are shown in Table 2.

Examples 3 to 7

The low molecular weight PTFE powder G obtained in Comparative Example 5was heated under the respective conditions shown in Table 2 using ahot-air-circulating electric furnace (Ultra-high temperature chamberSTPH-202M, Espec Corp.). Thereby, low molecular weight PTFE powders H toL were obtained. For the low molecular weight PTFE powders H to L, thephysical properties were determined in the same manner as in ComparativeExample 1. The results are shown in Table 2.

Reference Example 1

A low molecular weight PTFE powder was obtained by emulsionpolymerization in the presence of a chain-transfer agent in accordancewith Example 2 of WO 2009/020187. For the resulting low molecular weightPTFE powder, the number of carboxyl end groups was counted by infraredspectroscopy to be 7 groups per 10⁶ carbon atoms in the main chain.

Reference Example 2

A low molecular weight PTFE powder was obtained by emulsionpolymerization in the presence of a chain-transfer agent in accordancewith Preparation Example 2 of JP H08-339809 A, except that the amount ofethane added as a chain-transfer agent was changed to 0.22 g. For theresulting low molecular weight PTFE powder, the number of carboxyl endgroups was counted by infrared spectroscopy to be 15 groups per 10⁶carbon atoms in the main chain.

Comparative Example 6

Commercially available homo-PTFE fine powder (standard specific gravitymeasured in conformity with ASTM D4895: 2.175) was irradiated with 300kGy of cobalt-60 γ-rays at room temperature in the air. Thereby, a lowmolecular weight PTFE powder M having an average particle size of 31.6μm was obtained.

The physical properties of the resulting low molecular weight PTFEpowder M were determined. The results are shown in Table 3.

For the low molecular weight PTFE powder M, the number of carboxyl endgroups was counted by infrared spectroscopy to be 74 groups per 10⁶carbon atoms in the main chain.

Comparative Example 7

The low molecular weight PTFE powder M obtained in Comparative Example 6was pulverized using a pulverizer. Thereby, a low molecular weight PTFEpowder N having an average particle size of 5.7 μm was obtained.

The physical properties were determined in the same manner as inComparative Example 6. The results are shown in Table 3.

Comparative Example 8

The low molecular weight PTFE powder M obtained in Comparative Example 6was heated at 100° C. for 30 minutes using a hot-air-circulatingelectric furnace (Ultra-high temperature chamber STPH-202M, EspecCorp.). Thereby, a low molecular weight PTFE powder 0 was obtained.

The physical properties were determined in the same manner as inComparative Example 6. The results are shown in Table 3.

Comparative Example 9

The low molecular weight PTFE powder 0 obtained in Comparative Example 8was pulverized using a pulverizer. Thereby, a low molecular weight PTFEpowder P was obtained.

The physical properties were determined in the same manner as inComparative Example 6. The results are shown in Table 3.

Example 8

The low molecular weight PTFE powder N obtained in Comparative Example 7was heated at 100° C. for 30 minutes using a hot-air-circulatingelectric furnace (Ultra-high temperature chamber STPH-202M, EspecCorp.). Thereby, a low molecular weight PTFE powder Q was obtained.

The physical properties were determined in the same manner as inComparative Example 6. The results are shown in Table 3.

Example 9

The low molecular weight PTFE powder Q obtained in Example 8 was furtherpulverized using a pulverizer. Thereby, a low molecular weight PTFEpowder R was obtained.

The physical properties were determined in the same manner as inComparative Example 6. The results are shown in Table 3.

Comparative Example 10

A modified PTFE fine powder (amount of modification: 0.3%) (standardspecific gravity measured in conformity with ASTM D4895: 2.170) wasirradiated with 150 kGy of cobalt-60 γ-rays at room temperature in theair. Thereby, a low molecular weight PTFE powder S having an averageparticle size of 48.8 μm was obtained.

The physical properties of the resulting low molecular weight PTFEpowder S were determined. The results are shown in Table 4.

For the low molecular weight PTFE powder S, the number of carboxyl endgroups was counted by infrared spectroscopy to be 46 groups per 10⁶carbon atoms in the main chain.

Comparative Example 11

The low molecular weight PTFE powder S obtained in Comparative Example10 was pulverized using a pulverizer. Thereby, a low molecular weightPTFE powder T having an average particle size of 10.8 μm was obtained.

The physical properties were determined in the same manner as inComparative Example 10. The results are shown in Table 4.

Comparative Example 12

The low molecular weight PTFE powder S obtained in Comparative Example10 was heated at 100° C. for 30 minutes using a hot-air-circulatingelectric furnace (Ultra-high temperature chamber STPH-202M, EspecCorp.). Thereby, a low molecular weight PTFE powder U was obtained.

The physical properties were determined in the same manner as inComparative Example 10. The results are shown in Table 4.

Comparative Example 13

The low molecular weight PTFE powder U obtained in Comparative Example12 was pulverized using a pulverizer. Thereby, a low molecular weightPTFE powder V was obtained.

The physical properties were determined in the same manner as inComparative Example 10. The results are shown in Table 4.

Example 10

The low molecular weight PTFE powder T obtained in Comparative Example11 was heated at 100° C. for 30 minutes using a hot-air-circulatingelectric furnace (Ultra-high temperature chamber STPH-202M, EspecCorp.). Thereby, a low molecular weight PTFE powder W was obtained.

The physical properties were determined in the same manner as inComparative Example 10. The results are shown in Table 4.

Example 11

The low molecular weight PTFE powder W obtained in Example 10 wasfurther pulverized using a pulverizer. Thereby, a low molecular weightPTFE powder X was obtained.

The physical properties were determined in the same manner as inComparative Example 10. The results are shown in Table 4.

TABLE 1 Low molecular Amount Amount Average weight Irradiationconditions Treatment of of particle Melt Carboxy PTFE Temper- Atmos-after Heating PFOA PFC size viscosity groups powder ature phere Doseirradiation conditions (ppb) (ppb) (μm) (Pa · s) (N) Comparative A RoomAir 150 kGy — — 61 137 51.2 5.6 × 10⁴ 36 Example 1 temperatureComparative B Room Air 150 kGy Pulverization — 92 255 11.2 5.5 × 10⁴ 36Example 2 temperature Comparative C Room Air 150 kGy Heating 100° C./30min <5 <5 81.3 6.1 × 10⁴ 36 Example 3 temperature Comparative D Room Air150 kGy Heating, followed by 100° C./30 min 34 65 6.7 5.7 × 10⁴ 36Example 4 temperature pulverization Example 1 E Room Air 150 kGyPulverization, 100° C./30 min <5 <5 17.7 5.6 × 10⁴ 36 temperaturefollowed by heating Example 2 F Room Air 150 kGy Pulverization, 100°C./30 min <5 <5 3.9 5.6 × 10⁴ 36 temperature followed by heating,followed by further pulverization

TABLE 2 Low molecular Amount Amount Average Specific weight of ofparticle surface Melt PTFE Heating Heating PFOA PFC size area viscositypowder temperature duration (ppb) (ppb) (μm) (m²/g) (Pa · s) ComparativeG — — 249 644 2.2 8.9 3.5 × 10⁴ Example 5 Example 3 H 100° C. 3 hr <5 <52.7 9.0 4.5 × 10⁴ Example 4 I 150° C. 3 hr <5 <5 2.5 9.0 4.5 × 10⁴Example 5 J 200° C. 3 hr <5 <5 2.5 9.0 4.7 × 10⁴ Example 6 K 250° C. 3hr <5 <5 3.2 5.6 4.7 × 10⁴ Example 7 L 300° C. 3 hr <5 <5 5.1 6.4 5.0 ×10⁴

TABLE 3 Low molecular Amount Amount Average weight Irradiationconditions Treatment of of particle Melt Carboxy PTFE Temper- Atmos-after Heating PFOA PFC size viscosity groups powder ature phere Doseirradiation conditions (ppb) (ppb) (μm) (Pa · s) (N) Comparative M RoomAir 300 kGy — — 159 338 31.6 1.6 × 10⁴ 74 Example 6 temperatureComparative N Room Air 300 kGy Pulverization — 197 426 5.7 1.4 × 10⁴ 74Example 7 temperature Comparative O Room Air 300 kGy Heating 100° C./30min <5 <5 69.8 1.6 × 10⁴ 74 Example 8 temperature Comparative P Room Air300 kGy Heating, followed by 100° C./30 min 42 78 3.6 1.4 × 10⁴ 74Example 9 temperature pulverization Example 8 Q Room Air 300 kGyPulverization, 100° C./30 min <5 <5 15.7 1.4 × 10⁴ 74 temperaturefollowed by heating Example 9 R Room Air 300 kGy Pulverization, 100°C./30 min <5 <5 3.2 1.4 × 10⁴ 74 temperature followed by heating,followed by further pulverization

TABLE 4 Low molecular Amount Amount Average weight Irradiationconditions Treatment of of particle Melt Carboxy PTFE Temper- Atmos-after Heating PFOA PFC size viscosity groups powder ature phere Doseirradiation conditions (ppb) (ppb) (μm) (Pa · s) (N) Comparative S RoomAir 150 kGy — — 85 191 48.8 4.8 × 10⁴ 46 Example 10 temperatureComparative T Room Air 150 kGy Pulverization — 125 346 10.8 4.7 × 10⁴ 46Example 11 temperature Comparative U Room Air 150 kGy Heating 100° C./30min <5 <5 95.4 5.5 × 10⁴ 46 Example 12 temperature Comparative V RoomAir 150 kGy Heating, followed by 100° C./30 min 46 89 5.8 5.0 × 10⁴ 46Example 13 temperature pulverization Example 10 W Room Air 150 kGyPulverization, 100° C./30 min <5 <5 20.1 4.6 × 10⁴ 46 temperaturefollowed by heating Example 11 X Room Air 150 kGy Pulverization, 100°C./30 min <5 <5 4.4 4.4 × 10⁴ 46 temperature followed by heating,followed by further pulverization

The abbreviations in the tables represent as follows.

PFC: C8-C14 perfluorocarboxylic acids and salts thereof

PFOA: perfluorooctanoic acid and salts thereof

1. Powder comprising low molecular weight polytetrafluoroethylene, thelow molecular weight polytetrafluoroethylene having a melt viscosity of1×10² to 7×10⁵ Pa·s at 380° C., having an average particle size of 1.0to 50 μm, and containing 30 or more carboxyl groups at ends of themolecule chain per 10⁶ carbon atoms in the main chain, the powder beingsubstantially free from C8-C14 perfluorocarboxylic acids and saltsthereof.
 2. The powder according to claim 1, wherein theperfluorocarboxylic acids and salts thereof amount to less than 25 ppb.3. The powder according to claim 1, wherein the powder has a specificsurface area of 0.5 to 20 m²/g.
 4. The powder according to claim 1,wherein the powder has a specific surface area of 7.0 to 20 m²/g.
 5. Amethod for producing low molecular weight polytetrafluoroethylene,comprising: (1) irradiating polytetrafluoroethylene to provide lowmolecular weight polytetrafluoroethylene having a melt viscosity of1×10² to 7×10⁵ Pa·s at 380° C.; (2) pulverizing the low molecular weightpolytetrafluoroethylene; and (3) heating the low molecular weightpolytetrafluoroethylene pulverized in the step (2).
 6. The productionmethod according to claim 5, wherein the heating is performed at atemperature of 50° C. to 300° C.
 7. The production method according toclaim 5, wherein the heating is performed at a temperature of 50° C. to200° C.
 8. The production method according to claim 5, wherein thepolytetrafluoroethylene has a standard specific gravity of 2.130 to2.230.
 9. The production method according to claim 5, wherein both thepolytetrafluoroethylene and the low molecular weightpolytetrafluoroethylene are in the form of powder.
 10. The productionmethod according to claim 5, further comprising: (4) heating thepolytetrafluoroethylene up to a temperature that is not lower than theprimary melting point thereof to provide a molded article before thestep (1), the molded article having a specific gravity of 1.0 g/cm³ orhigher.
 11. Low molecular weight polytetrafluoroethylene obtainable bythe production method according to claim 5.